1. Field of the Invention
The present invention generally relates to a parallel monitor circuit and a semiconductor apparatus having such a parallel monitor circuit for monitoring one of serially connected electric double layer capacitors in order to equally recharge the capacitors, and specifically relates to such a parallel monitor circuit in which a monitor voltage for shifting the circuit to a standby mode can be varied.
2. Description of the Related Art
An electric double layer capacitor can be quickly recharged, compared with a secondary battery requiring a lot of time for recharging. Besides, the electric double capacitor has an advantage not attained by the secondary battery, in that it can store a great amount of energy. However, the electric double layer capacitor has a low rated voltage of 2.7 V, and therefore usually plural capacitors are serially connected to get a necessary high voltage.
Thus serially connected plural capacitors have a problem that recharging is not uniform or equal for each capacitor due to capacitance variation, self recharging, self discharging, etc.
In order to deal with the unequal recharging problem, a recharge equalizing circuit called “a parallel monitor circuit” is normally used.
FIG. 4 is a circuit diagram of a part of such a parallel monitor circuit disclosed in Japanese Patent No. 3,313,647.
Although FIG. 4 only shows one parallel monitor circuit, a plurality of serially connected parallel monitor circuits are usually employed. Each of the parallel monitor circuits is connected to one capacitor. The parallel monitor circuits have the same structure, therefore an explanation is given below with respect to only one parallel monitor circuit.
As shown in FIG. 4, the parallel monitor circuit comprises two reference voltage supplies Vr1 and Vr2, a comparator circuit CMP for comparing these reference voltages to a voltage of a capacitor C1, two switches S1 and S2 for switching connection to the reference voltages Vr1 and Vr2, a transistor Tr1 for bypassing the capacitor C1 with an electric current for recharging the capacitor C1, and a switch control circuit.
The reference voltage Vr1 is set at 3 V that is the same as the full recharge voltage for the capacitor C1. The reference voltage Vr2 is set at 0.8 V lower than the full recharge voltage. At the initial stage of recharging process, the switch S1 is connected to the reference voltage Vr2 (0.8 V). When the voltage of the capacitor C1 reaches 0.8 V, the output of the comparator circuit CMP is reversed to H, turning on the transistor Tr1. After the transistor Tr1 is turned on, the capacitor C1 is discharged with a time constant that is determined by resistance factors including the transistor Tr1.
The switching control circuit monitors the outputs of all the comparator circuits CMP. While the capacitor C1 is being discharged, if any other capacitor Cn reaches 0.8 V, the switching control circuit switches switch S1 to the reference voltage Vr1 to release the bypassing mode for the capacitor C1 and recharge the capacitor C1 up to the full recharge voltage 3 V.
In such conventional parallel monitor circuits, electric power is always supplied to the comparator circuit CMP even while the recharge control is not performed, resulting in wasteful power consumption, which is a problem.
In general, a standby mode is known for saving power to solve the above problem. In such a standby mode, while circuits included in a semiconductor apparatus are not required to operate, the circuit operation is stopped and supplied power becomes lower. However, in order to shift or transfer the semiconductor apparatus to the standby mode, it is necessary for the semiconductor apparatus to have a standby terminal, through which an external control circuit including a CPU supplies a standby signal to the semiconductor apparatus.
However, since the parallel monitor circuits receive control signals from the external control circuit for controlling the recharge of many capacitors connected thereto, they need many terminals. The number of terminals is very important when a semiconductor apparatus including such parallel monitor circuits is encapsulated in a general purpose package.
In more detail, in order to control step by step from the lowest IC to the highest IC among the serially connected semiconductor apparatus, two terminals are needed for one signal. When a plurality of status controls is necessary, two terminals for ON/OFF of one state bring high manufacturing costs, complex circuit layouts and a large area for the wiring board.
If the number of necessary terminals is more than the number of terminals of a general purpose package just by one, a larger and more expensive package must be utilized.